Most dictionaries list however as an adverb when it is used to mean ‘but’, ‘yet’ or ‘nevertheless’. In such contexts, careful punctuation is usually needed for clarity:

However, I will let you know.

I’m not sure of the outcome; however, I will let you know as soon as this is clear.

There is an increasing trend to use however as a conjunction joining two contrasting clauses, but with only a comma separating them:

I’m not sure of the outcome, however I will let you know as soon as this is clear.﻿

This use of however is not widely accepted, and should therefore be avoided in standard of formal publications.

Source: Style Manual, 6th edition.

Types of conjunctions

Coordinating conjunctions, also called coordinators, are conjunctions that join two items of equal syntactic importance. As an example, the traditional view holds that the English coordinating conjunctions are for, and, nor, but, or, yet, and so (which form the mnemonic FANBOYS).

Note that there are good reasons to argue that only and, but and or are prototypical coordinators, while nor is very close. So and yet share more properties with conjunctive adverbs (e.g. however), and ‘for…lack(s) most of the properties distinguishing prototypical coordinators from prepositions with clausal complements’. Furthermore, there are other ways to coordinate independent clauses in English.

Correlative conjunctions are pairs of conjunctions which work together to coordinate two items. English examples include both … and, either … or, not (only) … but (… also).

Subordinating conjunctions, also called subordinators, are conjunctions that introduce a dependent clause; English examples include after, although, if, unless, and because. Another way for remembering is the mnemonic ‘BISAWAWE’: ‘because’, ‘if’, ‘so that’, ‘after’, ‘when’, ‘although’, ‘while’, and ‘even though’.